Sounders and loud speakers may be used to alert persons that a hazardous situation exists in an area, such as in harsh environmental conditions found offshore and onshore in the oil, gas, and petrochemical industries. Sounders are usually used for reproduction of sound tones, such as fire alarms. Loud speakers are usually used for live speech production via an amplifier and a microphone. As used herein, the term “speakers” collectively refers to sounders and loud speakers. The design dilemma faced when designing a speaker for use in such hazardous areas is that speakers are fundamentally reliant on the unimpeded movement of air in and out of the speaker housing. However, hazardous area requirements dictate that the speaker housing must be sealed from the surrounding atmosphere to fully contain any possible sources of ignition within the housing, thus preventing propagation of an explosion. Therefore, speakers for use in hazardous areas must be designed to contain an internal explosion, while emitting sound (compressions in air), without compromising the strength and integrity of the speaker.
Currently, hazardous area speakers are based around a compression driver unit used to produce sound and disposed in an enclosure and a re-entrant horn used to provide impedance matching between air and a driver diaphragm, such as in a megaphone. The arrangement of the compression driver and the re-entrant horn is favored in the industry, as it allows the use of a sinter positioned in the throat of the re-entrant horn. As used herein, the term “sinter” refers to any sintered element that allows compressions of air (sound) to at least partially pass through, but also has the ability to remove some heat energy from a flame passing therethrough. The sinter should allow sound created by the driver to pass out of the enclosure and into the re-entrant horn to be amplified but also should “arrest” a flame. In other words, the sinter should have the ability to prevent flame transmission by removing one element (heat) of the combustion triangle (oxygen, fuel, and heat).
Conventional sinters are produced by pressing together sinter material, such as small bronze balls about 200 microns in diameter or stainless steel flakes, in a die to form a substantially circular, square, or rectangular component. The component is then heated to a temperature below the actual melting point of the material but at a high enough temperature to allow the sinter particles to fuse together. The sinter particles fuse together in the areas where they are in contact with each other. Ultimately, the fused sinter particles form a matrix of channels within the component, thus forming a sintered element. The sinter's ability to extinguish combustion results from the transfer of heat energy, or enthalpy, from the flame to the solid matrix of channels within the sinter. The rate of heat transfer depends on the temperature gradient between the flame and the sinter, the channel hydraulic diameter, and the thermal conduction (diffusivity) properties of the gas.
Several disadvantages to conventional sinters exist however. Since the sinter effectively acts as a heat sink, its own temperature increases as it conducts heat energy away from the flame. Eventually the temperature rise reaches a point where the sinter itself becomes an ignition source. This rise limits the ambient temperature range in which the product can be operated. Since the ability of a sinter to arrest a flame, and at the same time to allow sound to pass through, essentially depends on the relationship between pore size and flame path length, an increase in flame path length results in a corresponding nonlinear increase in pore size for a given degree of flame transmission protection under constant explosive atmospheric conditions. In other words, an increase in pore size compromises the sinter's ability to arrest a flame, while a decrease in pore size compromises sound emission or sound pressure level.
Additionally, when a conventional sinter is used in a hazardous area product, the maximum possible pore size must be specified and cannot be exceeded in production, since the largest pore is where a flame will pass through the sinter first. To achieve the requirement of not exceeding the maximum pore size during production of a sinter, a nominal pore size is specified where the manufacturing tolerance is such that the maximum pore size is not exceeded. The net result is that for a sinter with a certified maximum pore size of, for example, 250 microns, the nominal pore size may be around 200 microns and could be as low as 150 microns, resulting in lower sound pressure level through the sinter. The smaller the mean pore size, the lower the sound pressure level. Currently, the detrimental effect that the resultant mean pore size has on the sound output is simply accepted in the industry. Furthermore, the current process of manufacturing sinters results in a low yield of sinters (approximately less than 50% from a batch produced) that may be used in a hazardous area. Thus, the cost of manufacturing and testing usable sinters is high.
Therefore, a need exists in the art for a sintered element that can be economically produced while effectively arresting a flame without compromising sound pressure level.